This invention relates generally to machinery for screening paper-making pulp and more particularly to a screening apparatus having an enhanced rotor for promoting screening efficiency together with power conservation.
The quality of pulp is, to a large extent, determined by its freedom from contamination with shives, woody particles, dirt, and coarse fibers.
Shives and woody particles are usually small fiber bundles which are held together by a small amount of undigested lignin. Their presence to excess in commercial pulp results in reduced value and may cause downgrading of the pulp.
Dirt may be of organic or inorganic composition. Inorganics include small pieces of sand or other grit, while organics may be residual particles of bark or pitch agglomerations. Typically, these are visible to the unaided eye and are usually of a different color from the pulp. Since they all have various adverse effects, they also cause downgrading of the pulp and a consequent reduction of value.
The presence of any or all of these is undesirable, and many techniques for their separation have been developed with widely varying operating success records. These usually involve screening performed in a chamber in which relative motion is created between the screen and some agitation device such as a rotor or a hydrodynamic foil, one of which may be movable. The most common shortcomings of available separation machines are their inability to remove sufficient shives without also removing an undesirable quantity of good fiber; the tendency to block the screen with anything other than the lowest consistency of pulp; the tendency for the liquid component of the pulp suspension to pass more readily than the fibers through the screen, resulting in a progressive increase of consistency from the pulp feed to rejects discharge which results in unstable operation of the screening apparatus; the damage and wear caused to the screen and rotor by coarse particles caught between them; the consumption of excessive power for supplying the pulp and for screening; and the progressive damage to the pulp fibers caused by dissipation of the excessive energy used in the process.
Ideally, a screening apparatus would remove all shives and foreign particles from high consistency pulp without any of the good fibers being lost; without requiring any addition of dilution liquor; and without more than the minimum consumption of energy. In reality, employment of a screen with perforations fine enough to remove all undesirable material usually results in frequent screen blockages and possibly even jamming of the pulp between the rotor and the screen. Use of a screen coarse enough to prevent blocking and jamming usually results in accepting nearly all the undesirable particles. To avoid both of these objectionable results, screen aperture sizes are chosen to be slightly larger than that at which unstable operation occurs. To minimize unstable operation, agitation is provided to ensure that fibers pass individually through the screen apertures, and screenplate cleaning is typically provided by backwashing accumulated particles out of the apertures.
It has become quite common to use a screen/rotor combination for agitation and for hydrodynamic generation of backwash pulses in a fine screening apparatus. The aperture size, pulse generator form and size, rotor form and speed, and rotor/screen separation, for a given pulp type and consistency, determine the screening behavior. In some cases, positive pressure pulses may be too strong and too long relative to the negative pressure backwash pulses and may result in marginally rejectable fibers being extruded through the screen apertures and passing back and forth through the screen many times. This may damage the fibers by removing secondary fibrils and may result in paper having low strength. It also consumes excessive energy by causing multiple unnecessary screen passes for accepts fiber, which thereby reduces production capacity.
Some machines have a cage type rotor which may consist of one or more hydrodynamic foil members, mounted on support arms which radiate from a rotatable hub, extending axially for the full length of the screen. These generate strong pulses which sweep around the circumference over the full length of the screen with every revolution of the rotor. Such rotors consume excess power due to stirring action on the pulp residing inboard of the foil members. This power is wasted because it does not contribute sufficiently to the screening action.
One improvement to the cage type rotor provides a large diameter hub on which the hydrodynamic foils are mounted on short support arms to reduce the volume of the screening chamber and to reduce specific power consumption.
To reduce the magnitude of the effects described above, many machines are now made with closed rotors, that is, rotors having a full cylindrical surface on which bumps and depressions are directly attached without support arms to generate localized pressure pulsations. Depending upon their specific geometries, these may offer lower specific power consumption than cage rotors; and, because the bumps and depressions are distributed over the rotor surface, the pressure pulsations are distributed about the screen plate surface and do not concentrate alternating stresses along the aperture pattern.
Although these and other modifications have improved screening economics, there is still room for very significant improvement in screening before diminishing-returns sets in. For example, depending on the pulp characteristics in the particular pulp line, it is possible to tailor the numbers, shapes, locations, and sizes of the bumps and depressions to provide pulses of different durations, intensities, frequencies, and shapes which maximize screening efficiency and discrimination ability while reducing or minimizing power consumption.
Until these improvements are realized, fine screening efficiency will be less than desired, and pulp quality will suffer because of failure to remove a sufficient portion of objectionable material. In addition, the waste of good fiber due to its removal with rejects, the damage to fiber quality attributable to excessive mechanical working, and the excessive consumption of power (due to redundant stirring and to excessive dilution required to overcome thickening of the pulp) imposes a significant cost on the pulp making process.
The foregoing illustrates limitations known to exist in present fine screening apparatus. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.